The present invention relates to a method of reading on a magnetic recording medium and to apparatus for carrying out the method. It is more particularly applicable to magnetic disc memories.
In present day data-processing systems use is being more frequently made of magnetic disc memories, because of their storage capacity and the relatively short time required for the magnetic read/write heads to access information situated anywhere on the discs from the moment an instruction to access this information is received from the processing system.
It is known that magnetic discs carry information in coded form on concentric circular recording tracks whose width does not exceed a few hundredths of a millimeter and which are formed on both faces of the discs. The tracks are identified by allotting them a serial number j (j being a whole number) between 0 and (N-1), N being the total number of recording tracks. A coded expression of the serial number j of a track is called an address. The codes most frequently used are binary codes.
It is known that, for small storage capacity memories, which contain only one or two discs, the track addresses on one face of a disc are recorded on this same face in such a way that a maximum amount of disc space is set aside for recording data from the data processor and a minimum amount of space is set aside for address recording and for recording information required to servo-control the the magnetic head associated with the face in question to the contact track position.
In present practice, the addresses and the information for servo-controlling the head position are recorded sets of reference zones which are distributed over the entire surface of the disc. The number of zones is at least equal to the number of tracks, with each set of zones including indexing indicia which enable the beginning of the set to be found.
Each reference zone includes a predetermined number of cells, each of which contains an item of information for servo-controlling position and one bit of the address, as is described in the commonly assigned U.S. Pat. application Ser. No. 765,058 entitled "Method of Writing Addresses on a Magnetic Recording Medium", filed Feb. 2, 1977. The number of cells in a zone is equal to the number of address bits.
It will be recalled that the word "bit" refers both to a binary digit, 1 or 0, and to any way of expressing a binary digit either on a magnetic record, as an analogue single or an electric logic signal; a binary analogue signal is defined as a signal voltage that varies between a value (+V) and a value (-V), whereas a logic signal can assume only two values, known as "logic zero" and "logic one".
In the prior art, to record a succession of information items on a magnetic medium, each track of the medium includes a succession of small magnetic domains termed "elementary magnetic areas." These areas are distributed along the entire length of the track. The magnetism induced in successive areas is of opposing value, sense or polarity, and is orientated in a direction parallel to the disc surface. Each cell for positioning information and one address bit includes two magnetization changes. Each of these changes may occupy one of two predetermined positions within the cell. The first change constitutes the positioning information, while the second change, termed an "adjusting change," indicates the address bit value, as determined by the position occupied by the change. When a magnetic head encounters a series of magnetization sense changes representing a reference zone, it emits a series of analogue signals which are formed into a succession of logic pulses by shaping circuits. The beginning of the reference zone is indicated by a special pulse.
If the logic pulses representing the adjusting changes are considered, each change may occupy two positions. The time interval separating each adjusting change pulse from a pulse occurring at the beginning of the reference zone differs, depending on the value of the address bit.
An object of the invention is to provide a method of reading addresses on a recording medium by finding the time position of each logic pulse in relation to the pulse indicating the beginning of the zone and storing the address bits as binary logic zero or one binary signals.
This is achieved by frequency dividing a first signal of constant frequency termed the "reference signal to form ", a second signal which is synchronised by the logic pulses. The value of each address bit is obtained by finding the temporal position of each of the logic pulses within each period of the second signal.